Diodes are used in a wide range of electronic circuits. Diodes used in circuits for high voltage switching applications ideally require the following characteristics. When biased in the reverse direction (i.e., the cathode is at a higher voltage than the anode), the diode should be able to support a large voltage while allowing as little current as possible to pass through. The amount of voltage that must be supported depends on the application; for example, many high power switching applications require diodes that can support a reverse bias of at least 600V or at least 1200V without passing a substantial amount of current. When current flows through the diode in the forward direction (from anode to cathode), the forward voltage drop across the diode Von should be as small as possible to minimize conduction losses, or in other words the diode's on-resistance Ron should be as small as possible. Finally, the amount of charge stored in the diode when it is reverse biased should be as small as possible to reduce transient currents in the circuit when the voltage across the diode changes, since reduced transient currents result in reduced switching losses.
In diodes, there is typically a trade-off between the various characteristics described above. For example, silicon Schottky diodes can typically exhibit excellent switching speed and on-state performance, but suffer from large reverse leakage currents, making them unsuitable for high voltage applications. Conversely, high voltage Si PIN diodes can support large reverse bias voltages with low leakage, but typically exhibit high conduction and switching losses. Further, reverse recovery currents in PIN diodes add to these losses when the PIN diodes are incorporated into circuits.
FIG. 1 shows a cross-sectional view of a prior art III-N semiconductor heterostructure diode. As used herein, the terms III-N or III-Nitride materials, layers, devices, etc., refer to a material or device comprised of a compound semiconductor material according to the stoichiometric formula AlxInyGazN, where x+y+z is about 1. The diode structure includes a substrate 20, a first III-N semiconductor layer 22 on top of the substrate, and a second III-N semiconductor layer 24 on top of the first III-N layer. III-N layers 22 and 24 have different compositions from one another, the compositions selected such that a two-dimensional electron gas (2DEG) 26 (illustrated by a dashed line) is induced in the first III-N layer 22 near the interface between the first and second III-N layers 22 and 24. An anode contact 27 (or a plurality of anode contacts, not shown) are formed on top of surface 25 of the second III-N layer 24, and a single cathode contact 28 is formed which contacts the 2DEG 26. The anode contact 27 is a Schottky contact, and the single cathode contact 28 is an ohmic contact.
Anode and cathode contacts 27 and 28, respectively, may be any arbitrary shape, although the shape can be optimized to minimize the on-resistance Ron of the device. Further, the choice of metals for the contacts, especially that of the anode contact 27, can affect the forward operating voltage Von (also known as the on-voltage) of the device. It is desirable to provide diodes for which high blocking voltages and low reverse leakage currents can be achieved while at the same time maintaining lower on-resistance and control of the forward operating voltage. Diode structures which can easily be integrated with other circuit components, such as transistors, are desirable for process integration and cost reduction.